The invention relates to determining a sampling instant of a clock signal for a circuit for determining symbols from a received, specifically, complex modulated signal.
In the reception of digital signals coupled to a quadrature signal pair, the sampling frequency and sampling instant of a clock signal are a critical factor to ensure rapid locking-in of decision-feedback control loops. These control loops are found, for example, for the purpose of setting sampling instants when an equalizer is adjusted which eliminates linear distortions in reception of the quadrature signal pair, or during carrier-frequency or carrier-phase control.
The symbols thus received, and specifically, coded symbols, which form the actual data, represent a single-digit or multi-digit digital value in coded form. The coding is implemented for transmission via the quadrature signal pair which corresponds to a pointer that occupies discrete positions within the Cartesian amplitude and phase space of the quadrature signal pair at defined instants in time. These instants follow each other at equidistant intervals and must be hit as precisely as possible by the sampling clock. Conventional transmission methods of this type are Quadrature Amplitude Modulation (QAM) and Phase Shift Keying (PSK).
In a conventional receiver for receiving digital signals, a complex multiplier or mixer, which is controlled by a local oscillator, mixes the received QAM signal, modulated in-phase and in the correct frequency onto a carrier, into the base band of the circuit. With digital processing, this can occur before or after analog-to-digital (A/D) conversion. The signal is sampled either at the symbol clock or multiple thereof, or the digitization clock is free-running relative to the required symbol clock. In this last case, the signal is ultimately converted to the symbol clock or multiple thereof by a purely digital sampling rate conversion. Gain controls ensure that the relevant control range is utilized and that the received signals are correctly mapped to the symbol decider stage. An adaptive equalizer reduces intersymbol interference originating in linear distortions of the transmitter, transmission path, or receiver.
In high-order demodulators for QAM or PSK signals, the control circuits require the received signals as well as those elements of the predefined symbol alphabet viewed as the most probable by the decider stage for the frequency control and phase control of the local oscillator, for recovery of the symbol clock, and for the adaptive equalizer. This type of control via the decision-based symbol is called “decision-feedback” control.
Since the decision-feedback controls in prior-art digital demodulators are interconnected, locking in is difficult so long as the control for the carrier of the local oscillator mixing the received signal into the base band is no yet stable in regard to frequency or phase. Locking in often succeeds only whenever the given frequencies or phases are in relatively close proximity to their required values. Examples of decision-feedback controls are found in the text on basic principles by: K. D. Kammeyer, “Message Transmission,” [Nachrichtenübertragung], Verlag B. G. Teubner, Stuttgart, 2nd edition 1996, in chapter 5.7.3 “Adaptive equalizer with quantized feedback,” pages 200 to 202, in chapter 5.8.3 “Decision-feedback clock control,” pages 213 to 215, and in chapter 12.2.2 “Decision-feedback carrier phase control in the baseband,” pages 429 to 431.
Circuits designed as demodulators usually use one of two timing recoveries in order to supply a suitable sampling instant for the clock of the circuit. These involve either obtaining the clock signal by nonlinear distortion of the input signal, such as rectification or exponentiation, preferably, with the number of symbols, and subsequent bandpass filtering of the result at the expected symbol rate, or a decision-feedback timing recovery.
The first-named method is too imprecise, especially in the case of high-order modulation methods. The second method requires an in-phase baseband signal right from the start to enable the correct symbol decisions. For this purpose, carrier frequency and carrier phase control must have already locked in, an essentially impossible action, however, since due to the unknown sampling instant the symbols cannot yet be detected, and thus the decision-feedback control of carrier frequency and carrier phase cannot yet have locked in.
Therefore, there is a need for a system and method for determining the sampling instant of a clock signal for a circuit for determining symbols taken from a digitized signal which is coupled to at least one quadrature signal pair, and to improve a corresponding circuit.